1. Field
The present invention relates to a passive optical network (PON), and more particularly, to an optical network unit (ONU) for a multi-wavelength passive optical network (MW PON) and an operating method thereof.
2. Description of the Related Art
As optical communication technology is advanced and the demand for the Internet service increases rapidly, fundamental research on an optical access network has been conducted since the early 2000s, and thus introduction of a broadband convergence network (which directly connects an office or a central office (CO) to subscribers through an optical fiber) such as fiber to the home (FTTH) and fiber to the office (FITO) is generalized. Herewith, research on next generation super high-speed large-scale optical access network technology is being actively done for responding to an explosive increase in traffic due to the spread of mobile Internet protocol (IP) terminals such as smartphones or tablet computers, the commercialization of an IP television (IPTV) service, and the spread of a multimedia broadcast/streaming service over the Internet.
As a method for efficiently providing a service to more subscribers with limited network resources, a time division multiplexing (TDM) technique and a wavelength division multiplexing (WDM) technique are being applied to optical access network technology. Recently, research is being conducted on an optical access network using a hybrid technique in which both the TDM technique and the WDM technique are applied. Attempts to apply an orthogonal frequency division multiplexing (OFDM) technique (which is mainly used in wireless communication at present) to the optical access network technology are also being actively made, which is an example of the hybrid technique in a broad sense.
Among the techniques, the WDM technique or the hybrid technique may perform communication using a plurality of wavelength bands, namely, a multi-wavelength. As the use of the Internet increases and demand for multimedia contents increases explosively, increasing a bandwidth of a network in a wired optical access network and a wireless network or a merged wired/wireless network thereof is becoming an increasingly important issue, and particularly, a technique using a multi-wavelength is attracting an attention as a type of method for solving the important issue. According to this, it is possible not only to provide a super high-speed communication service to many subscribers, but also to easily expand a communication capacity and the number of subscribers with an excellent communication security. Therefore, in the next generation super high-speed large-scale optical access network technology, an MW PON using the WDM technique or the hybrid technique is obtaining a great interest.
An MW PON system may include a service provider (hereinafter referred to as “an optical line terminal (OLT)”) installed in a CO, a user terminal unit or a number of subscribers (hereinafter referred to as “an optical network unit (ONU)”) neighboring thereto, and a local node in which one or more optical multiplexers/de-multiplexers or light intensity splitters are installed or an optical distribution network (hereinafter referred to as “an optical distribution unit (ODN)”). In the MW PON system, a network configuration may be varied depending on the kind of used light source, for example, a spectrum-split light source, a wavelength-locked light source, or a wavelength-independent light source.
In a PON system, such as an MW PON system, one OLT and multiple ONUs exchange signals therebetween, and each ONU transmits an upstream signal thereof using resources allocated by the OLT. For example, in a TWDM PON system, each ONU is required to transmit an upstream signal using a specific wavelength at a designated time, wherein the wavelength and the time are allocated by the OLT, and in the MW PON system, so each ONU is required to transmit an upstream signal using a wavelength allocated at least by an OLT.
The use of a tunable transceiver has been actively considered as an ONU for the TWDM PON system or the MW PON system, in order to utilize a light intensity distributor-based ODN that has been used in the existing TDM PON and also to flexibly allocate wavelength resources. However, if the tunable transceiver fails to stably transmit an upstream signal over an allocated wavelength channel, the upstream signal may be directed to an undesirable OLT, that is, an OLT using a different wavelength, which may result in loss of upstream data.
To address the above drawback, the PON system detects ONUs that are likely to transmit upstream signals using unallocated wavelength channels and/or unallocated time slots, and then the PON system prevents the detected ONUs from transmitting upstream signals or separates them from the system. As another alternative, a research to develop a technology for a wavelength of an output signal from a tunable transmitter to remain stable is ongoing.
One proposed alternative method is to control an output wavelength while transmitting and receiving a calibration message with an OLT (S. Pachnick, et. al., “Investigation of wavelength control methods for next generation passive optical access networks,” ECOC2012, P6.02), in which an OLT receives an upstream signal transmitted from an ONU and uses a separate channel, such as a frequency or an out-of-band, in order to tune the upstream signal to a maximum value. However, according to this method, a system, such as a TWDM PON system, which also uses a time and wavelength division multiplexing mechanism, may not be able to detect a momentary drift of an output wavelength of a tunable transmitter.
As the second alternative method, an ONU autonomously aligns an output wavelength (Ning Cheng, et. al., “Automatic ONU wavelength control in TWDM-PONs,” OFC 2014 W1D.4). Specifically, a tunable transmitter of the ONU outputs a signal through a very low frequency band, and the ONU detects the degree of alignment of the wavelength and re-calibrate the wavelength based on an amount of reflected signal. However, in this method, a resolution may be degraded due to frequency band isolation in a wavelength band splitter.
The third alternative method is to use a wavelength locking device (S. Pachnick, et. al., “Investigation of wavelength control methods for next generation passive optical access networks,” ECOC2012, P6.02). According to this method, a centralized wavelength locker, which is costly, is placed at an OLT and stabilizes wavelengths of output signals from multiple tunable transceivers arranged at an ONU. However, feedback regarding the degree of alignment of the wavelength may be required to be provided from the OLT to the ONU, and this method may not be able to detect a momentary drift of a wavelength, which is also a drawback of the first method.
The fourth alternative method is to use a lookup table that has been precisely set (S. H. Lee, et. al., “Athermal Colourless C-band Optical Transmitter for Passive Optical Networks,” ECOC2010, Mo1.B.2). According to this method, however, since there are many variables need to be considered, such as ambient temperature, aging of laser, and hysteresis, which incur high cost to build up the precise lookup table and thus may cause an increase in price of a tunable transceiver.